Summary
Cluster, or star cluster, is the term used to refer to a specific combination of stars, their arrangement, the resource multiplier, and the Dark Fog difficulty settings. The cluster address combines the random number generator's seed, number of stars, resource multiplier, and dark fog difficulty separating each element by hyphens.
Aside: Random Number Generator (RNG) seeds are not specific to DSP, but briefly, when one sets an RNG to a seed, and then asks it for a random number, it will always generate exactly the same series of numbers. To create the illusion of randomness, seeds are usually set by the computer time; by setting a specific seed, the universe generation can behave identically for different players.
For the purposes of the Milky Way leader board, cluster addresses exclude the Dark Fog segment. Players playing without Dark Fog in an otherwise identical cluster (seed, stars, resource multiplier), will be scored together with players with Dark Fog.